<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        span {
            width: 25px;
            height: 25px;
            float: left;
            margin-left: 15px;
            background-color: black;
            color: white;
            text-align: center;
            line-height: 25px;
        }
    </style>
</head>
<body>
    <div>
        <span class="hour">1</span>
        <span class="minute">2</span>
        <span class="second">3</span>
    </div>
    <script>
        //1. 获取元素
        var hour = document.querySelector('.hour');
        var minute = document.querySelector('.minute');
        var second = document.querySelector('.second');
        var inputTime = +new Date('2023-2-8 15:00:00');
        countDown();//先调用一次这个函数 防止第一次刷新有空白
        //2.开启定时器
        setInterval(countDown, 1000);
        function countDown(){
            var nowTime = +new Date();//返回当前时间总的毫秒数
            var times = (inputTime - nowTime) / 1000;//times是剩余时间总的秒数
            var h = parseInt(times / 60 / 60 % 24);//时
            h = h < 10 ? '0' + h : h;
            hour.innerHTML = h;//把剩余的小时给 小时黑色盒子
            var m = parseInt(times / 60 % 60);//分
            m = m < 10 ? '0' + m : m;
            minute.innerHTML = m;
            var s = parseInt(times % 60);//当前秒数
            s = s < 10 ? '0' + s : s;
            second.innerHTML = s;
        }
        
    </script>
</body>
</html>